This invention relates generally to injection molding and more particularly to a valve gated injection molding system having an improved pneumatic valve member actuating mechanism.
Multi-cavity valve gated injection molding systems are well known having a valve member actuating mechanism with a piston which reciprocates in a cylinder mounted in the back plate. While this arrangement has been very satisfactory, it has the disadvantage that it requires a cylinder seated in the back plate and the cylinder requires a high pressure seal to prevent leakage of hydraulic fluid or air around the reciprocating valve pin. As described in the applicant's U.S. Pat. No. 4,740,151 which issued Apr. 26, 1988, it is also known to have a sealing and retaining bushing mounted on the manifold with a flanged portion bearing against the front surface of the back plate around the cylinder. A valve member bushing which is seated in an opening through the manifold and has a flange bridging the air space is shown in Gellert's Canadian patent application Ser. No. 604,608 filed June 30, 1989 entitled "Injection Molding System Having Dual Feed Bushing Seated in Manifold". However, while these previous bushings provided a seal and a containment chamber for leakage of melt past the reciprocating valve member, a separate high pressure seal was required for leakage from the cylinder.